Electrolysis is used to produce higher value chemical in different areas of the chemical industry, such as for the production of sodium chlorate, caustic soda and chlorine. Usually, the electrolysis takes place in an electrolyzer comprising an anode wherein oxidation reaction takes place, a cathode wherein a reduction reaction takes place, these two electrodes being separated by a an ion exchange membrane.
An electrolyzer is usually composed of an electrolyzing cell 2 comprising an anode 3 and cathode 5 (see FIG. 1). It is at the anode 3 that the oxidation takes place and at the cathode 5 that the oxidant is electrochemical reduced. Electrons are generated at the anode 3 and flow through an external load to the cathode 5. Ions flow between the anode 3 and the cathode 5 in an electrolyte to complete the circuit. A thin proton exchange membrane 7 enables the passage of the ions from the anodic compartment to the cathodic compartment.
In case of production of chlorine, saturated brine (sodium chloride, NaCl) is provided at the anode side of the cell where chloride ions (Cl−) are oxidized to chlorine (Cl2). At the cathode side of the cell, water is reduced to hydrogen (H2) and hydroxide ions (OH−). The latter combine with the sodium ions (Na+), migrating through the membrane from the anode side, to form caustic soda (NaOH).
The ways the anodes and cathodes are connected differ according to the technology. The electrodes can be connected in parallel, in series or in a combination thereof.
One of the problems associated with the monitoring of electrolyzing cells is the extremely hostile conditions in which they operate. This makes data acquisition difficult and unreliable. It is known, that the single cell voltage is without any delay responding to a malfunction of a cell. But the single cell voltage is also changing during normal operation, for example during a load change. Known single voltage monitoring systems are not precise and reliable enough to work as a safety system in a cell room and to cover the high risk class especially in case of cells, which are producing chlorine or/and hydrogen. Sometimes they are used to stop the process via the main process control system, if a certain high voltage level is reached. In practice the safety integrity level of these systems is not according to the risk level and the different kinds of malfunctions, which can be detected, is limited.
It is common, to install as a safety system a balance voltage monitoring system, which compares the average voltage of a group of cells with the average voltage of another group. This method is unreliable. During a short circuit for example one single voltage is reduced and the two single voltages of the neighbor cells are increased. Therefore the average voltage of the group is not changing.
It is also common to analyze the product quality to detect a malfunction of a cell. For example a defect membrane in case of the production of chlorine, caustic soda and hydrogen by the electrolysis of brine containing NaCl is resulting in an explosive mixture of hydrogen in chlorine. In most plants one analyzer is installed after the main chlorine cooler. Therefore in theory only an explosion outside the cell room can be avoided. But in practice also explosions in the chlorine treatment section happened, because of the response time of minutes of that analyzer, which is normally a gas chromatograph or a thermal conductivity detector.